1. Field of the Invention
The present invention relates to an improvement in frictional force transmitting devices used in a vacuum, which devices have frictional elements selectively pressed against each other so as to brake, accelerate or decelerate one of the frictional elements, or to transmit a frictional force from one of the frictional elements to the other.
2. Description of the Prior Art
For example, a conventional electromagnetic frictional brake, which is one of the typical frictional force transmitting device, generally has the following construction:
First, in this type of electromagnetic frictional brake, there are provided a braking side and a braked side.
Further, a braking-side disk and a braked-side disk are mounted on the braking side and the braked side, respectively. The braking-side disk is made of metal, while the braked-side disk is made of organic frictional materials.
In operation, the braking-side disk is urged against the braked-side disk to stop the same from rotating.
Incidentally, a disk made of organic frictional materials may be used as the braking-side disk.
On the other hand, in the above construction, the surface roughness of the metallic disk in the conventional electromagnetic frictional brake used in a vacuum is: 0.05 micron in Ra; and less than 2 microns in Rmax.
Surface roughness referred to above is publicly known as described in the following documents (1) and (2): document (1) shows it as Ra; and document (2) shows it as as Rmax:
Furthermore, it is known from the document (3) that studies on brake materials for vacuum/space brakes have been made up to the present.
(1) Hawthone, H. M: Wear Debris Induced Friction Anomalies of Organic Brake Materials in Vacuo, Wear materials, Vol.1 (1987), pp. 381-387;
(2) Iwata, Machida and Toda: Brake Materials for Space Actuators, Tribologist, Vol. 34, No. 10 (1989), pp. 757-764;
(3) Hawthone, H. M.: On the Role of Interfacial Debris Morphology in a Conforming Contact Tribosystem, 8th Int.Conf. on wear of Materials, ASME (1991), pp. 277-288.
The conventional device having the above-mentioned construction suffers from the following problem.
Namely, when an electromagnetic frictional brake is used in a vacuum, the brake's frictional force suddenly drops after it has performed sliding action over a certain length of distance, which often makes it impossible for the brake to further perform a predetermined braking function.
This phenomenon will be explained with reference to FIG. 8 which is a diagram illustrating the relation between the sliding distances and the coefficient of friction, and the horizontal axis of the diagram shows the sliding distance and the vertical axis shows the coefficient of friction.
The braking-side disks used in the experiments are made of metal (mild steel), and one of them has a surface roughness of 1 micron in maximum height (Rmax) and the other is free from such restriction placed upon the surface roughness, on the other hand, the braked-side disk is made of organic frictional materials.
In the diagram, the line "a" shows data obtained in the atmosphere, and the line "b" shows data obtained in a vacuum. As can be seen from FIG. 8, it is clear in a vacuum, the coefficient of friction drops suddenly when the cumulative sliding distance exceeds a certain value.
Although, the above-mentioned document (1) shows, as the same phenomenon, an example using a braking-side disk made of stainless steel and having a surface roughness of 0.05 micron in Ra, a problem arised when the example having such construction is applied to devices used in a vacuum.